As is known in the art, a phased array antenna is a directive antenna made up of a plurality of individual radiating antenna elements, which generate a radiation pattern or antenna beam having a shape and direction determined by the relative phases and amplitudes of the excitation signal associated with the individual antenna elements. By properly varying the relative phases of the respective excitation signals, it is possible to steer the direction of the antenna beam. The radiating antenna elements may be provided as dipole antenna elements, open-ended waveguides, slots cut in waveguides, printed circuit antenna elements or any type of antenna element.
The array antenna thus includes of a number of individual radiating antenna elements suitably spaced with respect to one another. The relative amplitude and phase of the signals applied to each of the antenna elements are controlled to obtain the desired radiation pattern from the combined action of all of the antenna elements. Two common geometrical forms of array antenna are the linear array and the planar array. A linear array antenna includes a plurality of antenna elements arranged in a straight line in one dimension. A planar array antenna is a two-dimensional configuration of antenna elements arranged to lie in a plane. The planar array antenna may thus be thought of a linear array of linear array antennas.
The linear array antenna generates a fan beam when the phase relationships are such that the direction of radiation is perpendicular to the array. When the radiation is at some angle other than perpendicular to the array, the linear array antenna generates an antenna beam having a conical shape.
A two-dimensional planar array antenna having a rectangular aperture can produce an antenna beam having a fan-shape. A square or a circular aperture can produce an antenna beam having a relatively narrow or pencil shape. The array can be made to simultaneously generate many search and/or tracking beams with the same aperture.
One particular type of phased array antenna in which the relative phase shift between antenna elements is controlled by electronic devices is referred to as an electronically controlled or electronically scanned phased array antenna. Electronically scanned phased array antennas are typically used in those applications where it is necessary to shift the antenna beam rapidly from one position in space to another or where it is required to obtain information about many targets at a flexible data rate. In an electronically scanned phased array, the antenna elements, the transmitters, the receivers, and the data processing portions of the radar are often designed as a unit.
In some applications, it is desirable to provide an antenna system capable of producing multiple, independent antenna beams. Such antenna systems are advantageous in a variety of different applications such as communication satellites, ECM, ESM radar and shared aperture antennas used to accomplish simultaneously a combination of these functions. In communication satellite applications, for example, the simultaneous objectives of relatively high EIRP (Equivalent Isotropically Radiated Power) and G/T (Gain over System Temperature), wide access footprints, channelized operation and a high spectral efficiency (i.e., frequency reuse) leads to the need for multiple, independent antenna beams. It is relatively difficult to provide an electronically scanned phased array antenna capable of producing multiple independent antenna beams due to the interaction between the signals of the multiple antenna beams and the complexity of the multiple beamformer circuitry necessary to produce such multiple independent antenna beams.
The requirement for the phase array designer is made even more difficult when the operating frequency is selected to have a relatively high operating frequency in the frequency range of 20 to 30 GHz, for example, due to the corresponding decrease in the spacing between the antenna elements required for operation at that frequency. The problem is further exacerbated when it is desirable to provide a compact antenna system operating at a relatively high frequency range since the relatively small spacing between antenna elements and the need to couple feed circuits to the antenna elements result in difficult packaging requirements.
One approach to provide an antenna system having a relatively high operating frequency and multiple independent antenna beams is to utilize a lens or dish antenna which includes a separate feed circuit for each separate antenna beam. However, such an approach is relatively inflexible and it is relatively difficult to change the directions of the individual antenna beams. Thus, there is a significant interest in phased array antennas and in particular in electronically scanned phased array antennas.
It would, therefore, be desirable to provide an antenna capable of producing multiple independently steered antenna beams and which is compact, relatively low loss, and which consumes a relatively small amount of power. It would also be desirable to provide an electronically scanned phased array antenna capable of steering multiple independent antenna beams.
It would further be desirable to provide an electronically scanned phased array antenna in which failure of one phase shifter only affects one antenna beam and the one antenna element associated with the antenna beam. It would also be desired to provide an antenna in which there is no cascading of the amplitude and phase errors of phase shifters included in the phased array antenna.